I doubt you'd be able to use two controllers at once, especially since most daydream software will only be designed for use with the one controller that ships with the unit. Would the phone even be able to distinguish which controller is which for two handed use?

Would the phone even be able to distinguish which controller is which for two handed use?

That should be easy to implement per application. Like an initial input setup "press button on left controller, now press button on right controller". Each BT device has a unique hash anyway. You simply enumerate the 2 in your code and it should be quite straightforward.